In a high-speed optical transmission system, dispersion is an important factor that limits the transmission distance of the system. Especially, for a 10G and higher speed single channel and broadband system, dispersion compensation is an important issue should be considered for optical transmission apparatus development. In a high-speed optical transmission system, usually there are two kinds of dispersion compensation: dispersion compensation fiber (DCF) used for broadband dispersion compensation, and dispersion compensation grating (DCG) used for single channel dispersion compensation. Of course, there are other technologies, such as pre-chirped manner, decision and feedback manner, etc.
At present usually a broadband DCF segmented dispersion compensation is applied for a dense wavelength division multiplexing (DWDM) system. When using a broadband or narrowband DCF for dispersion compensation, usually the DCF is connected to transmitting end of the optical transmitter, receiving end of the optical receiver or optical repeater. Advantages of this dispersion compensation method are as follow. Working band is wide, so the broadband dispersion can be implemented. Devices are not sensitive with temperature, so it is a low requirement for working condition. The technology is mature, so it is suitable for chain network or DWDM system having more wavelengths. The disadvantage of this dispersion compensation method is larger attenuation, so it is necessary to have optical amplifier to compensate optical power, and this will increase cost. In addition, usually urban network architecture is a ring network. High-speed services in a ring network have different distances and not all services are high-speed service, so not all services need to be compensated. If in an urban wavelength division system, there are only a few wavelengths needs to be compensated, then using DCF will have higher cost. Moreover, with this method, threshold-value of nonlinear optical effect is low, so input power of the fiber needs to be controlled; and there is larger polarization mode dispersion (PMD).
For network with a few wavelengths carrying high-speed service, DCG is deployed for dispersion compensation in order to reduce cost. The DCG deploys a single channel dispersion compensation module consisted of a chirped grating fiber plus a circulator. If a wavelength needs to make dispersion compensation, then an Optical Add/Drop Module (OADM) or a De-Multiplexer/Multiplexer (DEMUX/MUX) is used to filter the wavelength and the filtered wavelength is inputted to a dispersion compensation module for making dispersion compensation. After the filtered wavelength has been compensated, it is added or multiplexed to a transmission fiber though OADM or MUX. Mostly, this manner is applied in a more than 10G single wavelength high-speed SDH optical transmission system. Advantages of this manner are the insertion loss is small, nonlinear effect is not obvious and the technology is mature. Disadvantages of this manner are an inserted add/drop module will increase insertion loss and cost, especially it cannot make on-line dispersion compensation for a wavelength.
Besides, a pre-chirped manner and decision and feedback manner is immature. There is another manner of using electrical relay to make 3R regeneration of a signal, but it is too expensive.